After being relatively stable from 1960-1980, the prevalence of obesity in the United States has increased dramatically in the past 30 years. Reduced sleep duration has been identified as a lifestyle factor associated with the increased prevalence of obesity and weight gain. Weight gain, or positive energy balance, results from energy (which comes from food and drink) intake exceeding energy expenditure; therefore, potential pathways producing weight gain include increased energy intake without a compensatory increase in energy expenditure or reduced energy expenditure without a compensatory reduction in energy intake. Laboratory studies examining the effects of sleep loss on energy homeostasis show that reduced sleep is associated with changes in appetite-regulating hormones, daily energy intake (kcals), preference for foods high in carbohydrate content, and timing of meal-taking. Additionally, studies show that extended wakefulness due to sleep loss causes changes in resting metabolic rate, diet induced thermogenesis and physical activity, which all contribute to total energy expenditure. The objective of this application is to examine how chronic sleep restriction (when sleep is curtailed on consecutive nights) affects energy intake (measured using daily caloric intake; Specific Aim 1) and energy expenditure (resting metabolic rate and diet-induced thermogenesis measured using indirect calorimetry; Specific Aim 2) and will explore whether the effects of sleep restriction on weight gain and neurobehavioral performance (measured with the Psychomotor Vigilance Test) are related (Specific Aim 3, Exploratory). Data for these Aims will be collected using healthy adults participating in ongoing protocols in the Sleep and Chronobiology Laboratory at the University of Pennsylvania (Specific Aim 1: n=103; Specific Aim 2: n=64; Specific Aim 3: n=238). It is hypothesized that during sleep restriction, subjects will consume significantly more calories per day compared with consumption during baseline. Preliminary data also collected in the Sleep and Chronobiology Lab illustrates that there are significant gender and race differences in the amount of weight gained during sleep restriction protocols: males gain more weight than females and African Americans gain more weight than Caucasians. Therefore, it is hypothesized that there will be gender and race differences in the amount of energy consumed during sleep restriction. In addition to changes in energy intake, it is also hypothesized that resting metabolic rate and diet induced thermogenesis levels (components of energy expenditure) will be significantly decreased when measured following five nights of sleep restriction compared with levels measured after two nights of baseline sleep. The findings from this proposal will be critical in our understanding of the relationship between short sleep duratio and increased risk for obesity.